High tension bolts are widely used to couple steel structures in the fields of construction and civil engineering. As methods of tightening high tension bolts, nut rotation methods are widely used in the USA and Europe, and torque management methods are widely used in Korea and Japan. In a torque management method, five or more samples selected from the same type of bolts are tested in the field to determine a target torque, and then the bolts are tightened based on the determined torque. Torque management methods are convenient for field construction as compared to nut rotation methods, but are affected by field conditions such as temperature, humidity, and thread states of nuts and bolts. Practically, if a torque coefficient varies from 0.11 to 0.19 during tightening, an axial force may be only 58 of a target value.
Thereafter, construction specifications state that when nuts and bolts are coupled, nut rotation angles during main tightening should be checked (allowable range: 120°±30°) after primary tightening and marking. However, in construction fields in which several thousands to several tens of thousands of high tension bolts are tightened, it is practically difficult to mark each bolt, and thus checking is rarely carried out in accordance with specifications.
To address such limitations, many techniques of improving clamping axial forces of high tension bolts have been developed at home and abroad. For example, ultrasonic devices for determining bolt axial force, and devices for determining axial force by detecting the power consumption of an electric wrench during tightening have been introduced. There are also many other new techniques. However, such techniques may be uneconomical when applied to the field of construction, and thus, such methods are rarely used for practical applications. In the USA, special washers having protrusions are used. If a target axial force is satisfied, the protrusion of such a washer is plastically deformed to indicate completion of tightening. However, since importation prices of such washers are relatively high, such washers are rarely used in domestic fields. Recently, torque-shearing high tension bolts, which are configured to be fractured at ends thereof at a predetermined torque value to indicate completion of tightening, are widely used in domestic construction fields. However, in the case of torque-shearing high tension bolts, if a pintail thereof is prematurely fractured due to variations in a torque coefficient, a desired amount of axial force may not be obtained, and thus an additional action or management may be necessary.
Although a nut rotation angle is commonly an inspection item after tightening in most tightening methods such as nut rotation methods, torque management methods, and methods of using torque-shearing high tension bolts, marking is rarely carried out after primary tightening in the field, due to the reasons described above, and thus it is practically difficult to inspect nut rotation angles after the completion of tightening.